This invention relates to an improved process in which aliphatic hydrocarbons are upgraded in a high-temperature endothermic conversion process. In particular, the invention provides a method for directly transferring heat to a fluid-bed aromatization reaction zone while also increasing the yield of valuable products.
The problem of transferring heat to high-temperature fluid bed endothermic hydrocarbon upgrading processes has been an obstacle to their commercial development. Methods known in the art for transferring heat to the fluid-bed reaction zone include preheating the catalyst or positioning a heat exchanger in the fluidized catalyst bed.
To heat the reaction zone with preheated catalyst, the catalyst typically enters the reaction zone at a temperature above about 870.degree. C. (1600.degree. F.). Heating the catalyst to such temperature generally requires passing the catalyst through a combustion zone which, due to the presence of water vapor at high temperatures, causes permanent steam deactivation of the catalyst. On the other hand, the installation of a heat exchanger increases capital costs due not only to the expense associated with an alloy heat exchange bundle but also to expenses resulting from enlarging the reactor shell to maintain a given reaction zone volume. Moreover, operation of such a design is more costly due both to maintenance expenses as well as to increased energy consumption associated with relatively inefficient indirect heat transfer.
Further, direct heat exchange with inert solid particles requires that the particles be heated, transported to the reaction zone, mixed with the reactants and then separated from the products. Thus, it can be seen that a method providing the benefits of direct heat transfer to a high temperature endothermic fluid-bed reaction zone without the materials handling problems associated with solid particles would enhance the commercial viability of such conversion processes.